This invention relates to a method for preparing active oxide powders. More specifically this invention relates to an improved method for preparing active oxide powders which are high sinterable and suitable for preparing shapes having a high percentage of theoretical density.
The preparation of shapes and bodies from oxide powders which have a density which approaches 100% of the theoretical density of the oxides is very important to a number of different industries. For example, certain refractory oxides such as zirconia and hafnia, which may be stabilized by the addition of small amounts of other oxide powders, are under investigation for use as electrodes in the generator channel of a magnetohydrodynamic generator. For this purpose it is generally preferred that the electrodes or at least that portion of the electrode is contact with the MHD working fluid be as dense as possible in order to prevent penetration of the electrode by any of the seed materials which are highly corrosive and which may promote rapid electrode breakdown and disintegration.
Other oxides such as yttria have been proposed for use as electrical insulating materials for high temperature applications, for example, lining the wall of a fusion reactor or separating the electrodes within an MHD channel. For these applications, densities which approach 100% of theoretical are important to improve the physical characteristics and to maintain insulation integrity during operation of the high temperature devices.
A number of zirconia-, hafnia-, and thoria-rare earth ceramics are described in U.S. Pat. No. 3,640,887, Feb. 8, 1972, which are transparent. Uses suggested for these transparent ceramics include windows in furnaces, lenses, in high-temperature microscopes, lamp envelopes and laser application. While other requirements are important, it is also necessary that the powders be sufficiently active or sinterable in order to achieve the high densities required for transparency.
Active oxide powders are conventionally prepared by precipitation. For example, one or more compounds containing the desired cations are dissolved in an aqueous solution generally containing a mineral acid such as nitric or hydrochloric acid to prepare a feed solution. Hydroxyl ion in an aqueous solution such as ammonium or sodium hydroxide is added to neutralize the feed solution - and to add sufficient hydroxyl ion to precipitate the cation as the hydroxide, which forms as a gel. The gel is washed with water to remove any soluble compounds which may be present, and dried to remove the mechanical water, forming a dried hydroxide. The dried hydroxide is then calcined in air or under controlled conditions to form the oxide which is then carefully ground to a finely divided oxide powder suitable for use in preparing dense bodies or shapes. Mixtures of oxide powders may be prepared by coprecipitation of two or more cations dissolved in the feed solution.
While the described method produces ceramic oxide powders of sinterable quantity, high densities, that is those which approach 95% or better of theoretical, are generally either not possible or not possible with any degree of consistency between batches of material prepared at different times. This, of course, presents manufacturing problems when large numbers of shapes or bodies having a particular density or porosity are to be prepared from different batches of powders.